Forests

Forests are distributed across the spectrum of rural to urban environments, covering 896 million acres (including approximately 130 million acres in urban, suburban, and developed areas), or 33% of land in the contiguous United States, Alaska, and Hawai‘i. The structure and function of these forests vary considerably across the Nation due to differences in environmental conditions (for example, soil fertility; temperature; and precipitation amount, type, and distribution), historical and contemporary disturbances, and forest management and land-use activities.

Figure 6.1: Climate Change Effects on Ecosystem Services

Figure 6.1: Many factors in the biophysical environment interact with climate change to influence forest productivity, structure, and function, ultimately affecting the ecosystem services that forests provide to people in the United States and globally. Source: U.S. Forest Service.

Forests on public and private lands provide benefits to the natural environment, as well as economic benefits and ecosystem services (for example, water, fiber and wood products, fish and wildlife habitat, biodiversity, recreational opportunities, spiritual renewal, and carbon storage) to people in the United States and globally. Public forests are mostly managed for non-timber resources or for multiple uses; private lands owned by corporations are mostly managed for timber production, whereas private lands owned by individuals are typically managed for multiple uses. To date, assessments of climate change vulnerability and development of adaptation options in the western United States have occurred mostly on public lands, whereas assessment and adaptation planning and implementation in the eastern United States span public and private lands, with documented examples of adaptation on most ownership types.19,20 The ability of U.S. forests to continue to provide goods and services is threatened by climate and environmental change and associated increases in extreme weather events and disturbances (for example, drought, wildfire, and insect outbreaks; Figure 6.1), which can pose risks to forest health (that is, the extent to which ecosystem processes are functioning within their natural range of historic variation)9 and conditions across large landscapes for years to centuries.1

The effects of climate change on forests in specific regions are discussed in many of the regional chapters (for example, Ch. 18: Northeast, KM 1 and 2; Ch. 19: Southeast, KM 3 and 4; Ch. 21: Midwest, KM 2; Ch. 24: Northwest, KM 1; Ch. 25: Southwest, KM 2; Ch. 27: Hawai‘i & Pacific Islands, KM 2 and 5). Rapid changes have been driven by severe drought in combination with insect outbreaks, which have killed more than 300 million trees in Texas in 201121 and more than 129 million trees in California from 2010 to 2017.22 Also, mountain pine beetles have caused tree mortality across more than 25 million acres in the western United States since 2010, representing almost half of the total area impacted by all bark beetles combined in that region. Recent warming has allowed mountain pine beetles to erupt at elevations and latitudes where winters historically were cold enough to keep them in check.4,23,24 Wildfire burned at least 3.7 million acres nationwide in 14 of the 17 years from 2000 to 2016—an area larger than the entire state of Connecticut—including a record 10.2 million acres in 2015 (an area greater than Maryland and Delaware combined). Over this same time span, annual federal wildfire suppression expenditures ranged from $809 million to $2.1 billion (Figure 6.4).

Recent insect-caused mortality appears to be far outside what has been documented since Euro-American settlement3 and is likely related to climate change. It is unclear if the apparent climate-related increase in area burned by wildfire is outside the range of what has been observed over centuries of fire occurrence.5 Drought, heavy rainfall, altered snowpack, and changing forest conditions are increasing the risk of low summer streamflow, winter flooding, and reduced water quality, with potential negative impacts on aquatic resources and human communities.12,13 A changing climate and forest disturbances also interact with chronic stressors (such as fungal pathogens and nonnative species) to affect the scale and magnitude of forest responses to climate change.25,26

The ability of society in general and resource managers in particular to adapt to climate change will be determined primarily by socioeconomic factors, technological developments, and organizational capacity (Ch. 28: Adaptation). Although some general principles apply to adaptation (defined here as adjustments in natural systems in response to actual or expected climatic effects that moderate harm or exploit benefits) across all forests, it is biophysical variability, socioeconomic conditions, and organizational objectives that dictate local management approaches. A viable forest-based workforce in local communities can facilitate timely actions that minimize the negative effects of climate change, as long as this workforce can support the objectives of treatments aimed at building forest resilience and provide a justification for treatments (for example, prescribed fire—the purposeful ignition of low-intensity fires in a controlled setting) that help minimize potential economic loss. Reduction in forestland associated with human land-use decisions, especially conversion of forests to nonforests on private lands, is a significant impediment to providing desired ecosystem services from forests. Hence, ensuring the continuing health of forest ecosystems and, where desired and feasible, keeping forestland in forest cover are key challenges for society.